The answer depends on the reason why you are flying your friend and his employees around. You don’t state whether or not your friend employs you in his company, but from the wording of your question I would guess that is not the case.
IF you were employed by your friend, and IF the flight was “only incidental to that business or employment, AND the aircraft does not carry passengers or property for compensation or hire” then you may act as pilot in command AND receive compensation. 61.113(b), (1) & (2) This is the only scenario where a Private Pilot may actually be compensated.
However, if you are flying them around for fun, “out of the goodness of your heart”, then you would fall under 61.113(c) and would need to “not pay less than the pro rata share of the operating expenses.”
Don’t mix the subsections of 61.113 together, they describe distinctly different scenarios. Being the recipient of free flight time is not the same thing as being compensated to fly. Reference subsections (d) and (e).
For example, the Civil Air Patrol has many members with a PPL who join not only out of a sense of Civic duty, but to be able to fly without the costs of renting or owning an airplane. They are not compensated for their volunteer service, but if they are flying a CAP funded mission they receive free flight time.